Electrical wall switches and dimmer switches are known in the art which include illumination means, such as a neon lamp 16, for illuminating the switch while the load to be controlled, such as a lighting fixture is off, i.e., drawing minimal power. Dual switches are known for separately controlling the power delivered to each of two electrical loads to be controlled. Dual switches may comprise common on/off switches (e.g., SPST), hereinafter referred to interchangeably as switching devices, or more complex devices which include dimmer mechanisms or the like. Particular dual switches are known to include a separate neon lamp 16 or illumination means for each switching device within the switch. When one of the two switching devices is on and the other is off, the neon lamp 16 associated with the switching device that is off is illuminated, enhancing a person's ability to identify the location of the electrical wall switch in a darkened room. The neon lamp 16 associated with the switching device that is on, i.e., in the closed state, remains off.
Also known are dual electrical switches which include two switching devices and a single illumination means, e.g., a single neon lamp, for illuminating the dual switch when both switching devices are set to their off state. A schematic diagram illustrating a dual switch single illuminator circuit of the prior art with both switches in the open position is shown in FIG. 1A. The prior art circuit shown in FIG. 1A depicts a simple, dual switch on/off type lamp control circuit 10. The circuit comprises a single neon lamp 16 and two switching devices 12, 14 which separately control power to lamps 22, 24. Switching devices 12, 14 are electrically connectable at line ends to a phase line of an AC source (not shown) via the dual switch line terminal labeled AC-HOT. Terminal AC-HOT is also electrically connected to a first end of neon lamp 16. Second end of neon lamp 16 is electrically connected to the load end of switching device 12 and the line end of lamp 22 through a first resistor 18. Similarly, the second end of neon lamp 16 is electrically connected to a load end of switching device 14 and a line end of lamp 24 through a second resistor 20. Return ends of lamps 22, 24 are electrically connectable to a neutral terminal of the AC source via the terminal labeled AC-NEUTRAL.
A schematic diagram illustrating a dual switch single illuminator circuit of the prior art with one switch in the closed position is shown in FIG. 1B. With reference to FIGS. 1A and 1B, operation of the dual switch circuit 10 will now be described. When both switching devices 12, 14 are set to an off or non-conductive state, whereby power to lamps 22, 24 is reduced, and the break-over voltage in the range of 60 to 80 volts appears across neon lamp 16 and a parallel combination of two series circuits. The first series circuit is composed of resistor 18 and lamp 22 and the second series circuit is composed of resistor 20 and lamp 24 and the lamp 16 glows at full brightness. Note that the current supplied to lamps 22 and 24 from neon lamp 16 is not sufficient to make their filaments glow. If either of switching devices 12, 14 is switched to the on or conductive state, whereby power is delivered to one of the lamps, circuit of FIG. 1A can be redrawn as shown in FIG. 1B. In FIG. 1B, switch 12 is shown closed but the operation is identical if switch 14 is closed instead.
With switch 12 closed, there now exists a voltage divider comprising resistors 18, 20. Note that the resistance of lamp 24 is small compared to the resistance of the two resistors. Therefore, the voltage across the neon lamp 16 can reasonably be approximated to be one half the AC supply voltage or about 60 V. Assuming that a typical neon lamp 16 requires approximately 80 volts to light, the 60 V is below the striking potential, i.e., insufficient to light the neon lamp 16, and the neon lamp 16 is effectively an open circuit. Note that the currents and voltages are the same for both positive and negative half cycles of the AC voltage.
The neon lamp 16 remains off until that time at which both switching devices are again set to an off state, again establishing a current path through the neon lamp 16. Similarly, when both switching devices 12, 14 are set to an on state, the neon lamp 16 will not be energized.
Note that the above description assumes an AC input voltage of 120 V such as in use in the United States. If the same circuit of FIG. 1A is utilized in a country with a higher AC voltage, e.g., 220 V provided in most European countries, the neon lamp 16 would light. In this case, the current flowing through the neon lamp 16 is reduced with one switch closed compared with the current when both switches are open. The current is reduced because the current through the resistor 20 splits between the neon lamp 16 and resistor 18. Whereas with the switch 12 open, the current to the neon lamp 16 is supplied from resistors 18, 20 essentially in parallel.
While it is advantageous to use only one lamp in the dual-switch circuit because, for example, reduced manufacturing and operational cost (i.e., power consumption), a disadvantage is that the neon lamp 16 is effectively an open circuit and does not light when one of the switching devices is set to a conductive, i.e., on, state. For example, in a case where the two electrical loads controlled by the two switching devices are located in different rooms, both the neon lamp 16 and one of the two controlled loads could possibly be located in a room which is darkened, while the other load is energized elsewhere. Because the neon lamp 16 would not be illuminated, the light switch location in the darkened room would therefore be rendered difficult to identify.